Sen. Rand Paul prefers exhausting diplomatic efforts in dealing with overseas conflicts, and on Sunday he highlighted the difference between his softer approach to foreign policy and that of two other senators who are likely 2016 presidential candidates -- Ted Cruz and Marco Rubio.

The Kentucky Republican joined fellow first-term senators Cruz of Texas and Rubio of Florida on stage Sunday evening in California for a summit organized by Freedom Partners. That group is the central hub of the powerful network of organizations backed by conservative billionaires Charles and David Koch.

Each likely candidate has broad backing from tea party activists, who helped push them to victory over establishment-minded rivals in their most recent races. But a split is already starting to emerge even before they formally decide if they will run, and Paul seems to be an eager wedge.

"I'm a big fan of trying the diplomatic option as long as we can," Paul said of talks with Iran over its nuclear plan. "I do think diplomacy is better than war."

Lawmakers from both parties are pushing for a new round of sanctions against Iran. The White House and foreign leaders have urged Congress to not do that, for fear it would agitate Iran and prompt them to end negotiations over its nuclear abilities.

Cruz and Rubio were sharply critical of negotiations, backed by President Barack Obama.

"This is the worst negotiation in the history of mankind," Cruz said, predicting an Iranian nuclear strike in "Tel Aviv, New York or Los Angeles."

Added Rubio: "At this pace, in five years, we're going to build the bomb for them."

Paul urged his colleagues to have patience. "Are you ready to send ground troops into Iran?"

Cruz was having none of it. "The problem with Iran is Khomeini and the mullahs are radical Islamic nutcases," he said.

It was as intense a disagreement on Cuba. Obama late last year sent shockwaves across the hemisphere by restoring diplomatic ties with Cuba after more than a half-century of estrangement and embargoes. Politicians from corner of both parties were critical.

Cruz and Rubio, both Cuban-Americans fiercely opposed to the Castro regime's hold on power there, have been outspoken critics of Obama's move, while Paul notes the embargo has not ousted Fidel or Raul Castro.

"I'm kind of surrounded on this one," Paul said, sitting between Cruz to his right and Rubio to his left.

"The Castros are brutal dictators," Cruz said. He also said the potential for U.S. dollars flooding into Cuba would only keep the Castro regime in power longer.

"Maybe. Maybe not," Paul said.

Even on military spending, which is typically sacrosanct among Republicans, Paul needled his colleagues. Paul said national security is the most important spending in the budget, but "I'm not for a blank check."

Rubio said the United States' economic challenges did not stem from defense spending and smaller budgets would only threaten its future economic growth.

"Try economic growth while you're under attack," Rubio said.

The trio of lawmakers is laying the groundwork for presidential bids that are expected to launch in the coming months and will be competing for many of the same donors, including those the Kochs count as allies and who joined the weekend summit in Palm Springs, California.

The Koch network, which includes Americans for Prosperity, Generation Opportunity and the Libre Initiative, is unlikely to formally back one of the presidential hopefuls but its deep pockets can certainly focus the terms of the debate.

Sunday's event was closed to journalists but Freedom Partners broadcast the panel discussion with the three senators online for reporters, an unusual step toward transparency at the historically private gatherings. An earlier session with Wisconsin Gov. Scott Walker, another likely contender, was not available online.

"There are a bunch of Democrats who have taken as their talking point that the Koch brothers are the nexus of all evil in the world," Cruz said, acknowledging the event's organizers.

"I admire Charles and David Koch," Cruz continued. "They are businessmen who have created hundreds of thousands of jobs and they have stood up for free market principles."

The evening session's online broadcast did not include images of the audience so it was impossible to know if either Charles or David Koch were in the audience.